A recent development in wireless communications is the deployment and use of heterogeneous networks (sometimes called Het-Net), in which different cell sizes cover the same geographic region, using different base stations and downlink power levels. A conventional cell, served by a base station or eNodeB (eNB), is referred to as the macro cell. “Embedded” within the coverage area of the macro cell, one or more smaller cells may serve users within specific locations, such as within a campus, building, arena, or the like. The smaller cells are variously referred to as micro, pico, or femto cells (additionally, relays and/or repeaters may also be deployed within the macro cell). The term “micro” is used herein to refer to all such smaller, embedded cells. The base stations of these micro cells have different physical sizes and antenna arrangements, and transmit at a lower power, than the macro cell eNB, resulting in smaller geographic coverage areas. For example, the output power difference between a macro cell eNB and micro cell base station can be 10-20 dB, or even more. In the uplink, transmission power is, at least in principle, independent of the base station type and depends primarily on the User Equipment (UE) power capability and estimated channel quality to the receiving basestation (although in practice factors such as receiver sensitivity, number of receive antennas, antenna gain, and the like impact the required uplink power).
The goal of heterogeneous networks is to provide high data rates to localized users in micro cells within a macro cell. The higher data rates result from a better radio link due to close physical proximity to the micro cell base station antennas. By offloading users from the macro cell to the micro cells, the remaining UE served by the macro cell can be scheduled more resources, and hence can also achieve higher data rates. This yields both higher and more uniform data rates in the system.
Within 3GPP LTE Rel. 9, cell selection is based on the power of the reference, or pilot, symbols as measured by the UEs. Therefore, micro nodes have smaller coverage areas than the macro nodes which typically have a much higher transmit power. In later releases of LTE, e.g., probably release 10 and forward, it will be possible to extend the range of the micro cells by using a cell-specific cell selection offset. By increasing this offset, the micro nodes can serve more users.
However, this means that the control channel interference situation may be difficult. For example, the Physical Downlink Control Channel (PDCCH) from a micro cell base station may be severely interfered by an overlaying macro node. We assume herein that all cells, large and small, use the same frequency (band) and carrier.
LTE Rel. 10 will include functionality for time-domain inter-cell interference coordination (ICIC). This functionality includes a concept called Almost Blank Subframes (ABS), which in this context means that a macro eNB 12 will transmit with reduced power and/or activity on some physical resources during certain pre-determined subframes. This way, the interfered users in the embedded or neighbor micro cell will have some subframes during which the interference is low enough for the micro base station 16 to serve them with acceptable performance.
The subframes with reduced power and/or activity repeat according to a predetermined pattern. The pattern is 40 subframes long (40 ms) for LTE frequency division duplex (FDD). For LTE time division duplex (TDD), the pattern length is 20, 60, or 70 subframes long, depending on UUDL configuration. The pattern repeats for the entire Sequence Frame Number (SFN) period of 10240 subframes. The patterns are constructed in such a way that a protected subframe in the DL corresponds to subsequent Physical Hybrid ARQ Indicator Channel (PHICH) transmission occasions, for example if the DL control transmission is a UL grant.
Due to ABS, interference may vary significantly from subframe to subframe for a UE. The interference may be high in non-ABSs, and low in ABSs. In certain situations, a UE can only be scheduled during ABSs, due to the otherwise excessively high interference on downlink control and data channels. Accordingly, a UE should not include non-ABS subframes in CQI measurements and reports, as the measured interference would be higher than what the UE will actually experience. Measurement Restrictions is a feature to exclude subframes from measurements. The eNB signals to the UE which subframes to include in some measurement, such as CQI. This is specified in 3GPP TS 36.331.
The LTE TDD 10 ms frame comprises two half frames, each 5 ms long. The LTE TDD half-frames are further divided into five subframes, each 1 ms long. These subframes may be allocated to uplink, downlink, or “special” communications. Special frames include pilot signals and a guard period. The balance of uplink and downlink subfields, also known as TDD time slots, may be dynamically altered to meet load conditions. In order to provide for orderly alteration in the allocation of uplink and downlink subfields, LTE TDD defines seven subframe allocation configurations, numbered 0-6.
In the case of LTE TDD configurations 1-5, the ABS pattern has a period of 40 or 20 ms, and in either case, is an integer multiple of the SFN period length of 10240 ms. For LTE TDD configurations 0 and 6, however, the period of the ABS is 70 or 60 ms, leaving either a 20 or 40 ms “tail” of uncompleted ABS pattern at the end of each SFN period. For the subsequent SFN period, the ABS pattern is simply reset, and begins anew. The resulting ABSs at the beginning of the new SFN period are different than those that would result from the continuation/completion of the prior ABS pattern. This can cause significant interference problems on the downlink control channels if a UE uplink transmission, occurring within the tail period, requires multiple HARQ retransmissions.